Longitudinal studies have contributed much to the understanding of aging. Traditionally, age-specific changes in physiological functioning are inferred from studies in which persons with disease processes believed to be relevant to the function in question are excluded from the results. The main theme of this review is that, in the future, studies of aging must better attempt to capture the interplay between disease and aging processes. A variety of research results, mostly from the Baltimore Longitudinal study of aging, are used to support this argument. Topics covered include: the role of longitudinal studies in aging research; the complexity of the aging process; the significance of cohort and secular changes for understanding both aging and disease processes; age changes over the adult life span in the significance of risk factors for disease; and age-related increases in morbidity in a longevous group of men